1. Field of the Invention
The invention relates to a solid electrolyte-electrode system for use in an electrochemical cell. More particularly, the invention relates to a solid oxide electrolyte-electrode system for use in a solid state electrochemical cell.
2. Description of the Prior Art
Electrochemical devices, particularly solid state electrochemical devices such as those used for energy conversion and storage, gas sensing, and gas separation and purification require optimized and compatible solid electrolyte and electrode materials. Different, specific demands are placed on the electrical conductivity properties of electrolyte and electrode materials making up those devices.
The total conductivity of a material, due to both ionic and electronic charge carriers, is equal to the sum of its ionic and electronic conductivity. The fraction of the total conductivity carried by ions is referred to as the ionic transference number and the fraction of the total conductivity carried by electrons is referred to as the electronic transference number.
Solid electrolyte materials should be primarily ionically conductive and have ionic transference numbers close to unity and have electronic transference numbers close to zero to avoid cell discharge under open circuit conditions due to gas permeation. Ionic conductivity should be high to maximize cell voltage and minimize resistive losses. Typically, electrolytes meeting the above criteria and having sufficiently high conductivities to be useful in practical electrochemical devices have oxygen ions or hydrogen ions, protons, as the ionically conductive species. For oxygen ion conductor solid electrolyte materials, the ionic transference number must remain near unity over a wide range of oxygen concentrations as measured usually in terms of oxygen partial pressure. Furthermore, the electrolyte must be chemically and mechanically stable over the service temperatures at which the electrochemical device is to operate.
Solid electrodes, like solid electrolytes, must be chemically and mechanically stable at electrochemical device service temperatures and device environments. Additionally, solid electrodes must be chemically compatible with the solid electrolytes with which they are used to avoid formation of chemical reaction products which can compromise electrochemical device operation.
Special requirements are placed on solid electrodes used in fuel cells where oxidation and reduction reactions occur respectively at the anode and cathode electrodes. In order to achieve rapid reaction kinetics and avoid losses caused by overpotentials at the electrodes, ionic and electronic carrier species and gas phase species must be rapidly supplied to or removed from the electrode/electrolyte interface. Rapid kinetics and loss prevention is accomplished either by using electronically conductive electrodes with sufficient porosity to allow gas molecules to reach the three phase (electrolyte-reactant-electrode) interface or preferably by using an electrode characterized by mixed ionicelectronic conduction (MIEC) whose surface is everywhere catalytically active.
Conventional electrochemical cells typically use stabilized zirconia (e.g. ZrO.sub.2 stabilized with 9 mol % Y.sub.2 O.sub.3) as the solid electrolyte because of its excellent stability with respect to oxidation and reduction and high oxygen ion conductivity (.sigma.=10.sup.-1 S/cm at 1000.degree. C.) along with doped LaMnO.sub.3, and doped LaCrO.sub.3 and Ni--ZrO.sub.2 cermet solid electrode materials. When the electrochemical device is a fuel cell, perovskite crystal structured LaMnO.sub.3 LaCrO.sub.3 oxides serve as the air electrode and interconnect and a Ni--ZrO.sub.2 cermet is the fuel electrode. Such conventional zirconia based electrolyte and electrode systems are described in Japanese Patent No. JP 1288759 dated Nov. 21, 1989; U.S. Pat. No. 4,233,142 to Rohr et al., issued Nov. 11, 1980; and U.S. Pat. No. 4,412,904 to Rohr et al., issued Nov. 1, 1983.
Conventional zirconia electrolyte based electrochemical cells have several limitations among which are the following. Efficient cell operation is limited to approximately 900.degree.-1000.degree. C. resulting from the high oxygen ion conductivity activation energy of 0.9-1.0 ev characteristic of zirconia based materials which results in sharply decreasing electrolyte ionic conductivity with temperature and interfacial electrode resistance which rapidly increases at low temperature. Furthermore, the cell lifetime of a zirconia electrolyte based cell is limited as a result of chemical reactions between the zirconia electrolyte and the LaMnO.sub.3 and LaCrO.sub.3 electrodes to form resistive intermediate phases; cracking due to thermal-mechanical stresses that arise between the different materials in contact at the electrolyte-electrode interface; densification of porous electrodes thus limiting gas access to and removal from the electrolyte-electrode interface; and aging of the non-equilibrium cubic fluorite phase of stabilized zirconia resulting in ordering and trapping of the oxygen charge carriers thereby resulting in reduced electrolyte conductivity.
Thus, there exists a need for a stable electrolyte-electrode system capable of operating at temperatures in the range at from about 900.degree.-1000.degree. C. or preferably lower than the operating range of conventional zirconia electrolyte-based technology; minimizing stability difficulties resulting from chemical reactions and thermal mechanical stresses between the different compounds and aging of the electrolyte; and exhibiting enhanced electrode kinetics.